This invention is directed to multifunctional acyclic monomers which are cured to provide adhesives, coatings and restorative materials, and when cured, affix or coat electronic components or affix optical components, e.g., lenses and prisms, in assemblies.
Multifunctional (meth)acrylates have been broadly used as photopolymerizable resins in a wide range of applications including adhesives, coatings, restorative materials, information storage systems and stereolithography. Highly cross-linked networks formed from these resins have desirable properties for these applications such as high strength, and very good moisture resistance and the networks are formed by rapid curing. A conventional monomer is hexane diol diacrylate (HDODA) which has the structure 
When a conventional resin is used to affix or coat an electronic component in an assembly and the assembly becomes inoperative, it is impossible to repair the assembly by replacing an inoperative component or to recover or recycle operative components of an inoperative assembly. Moreover, conventional resins are unuseful to temporarily fix components, e.g., optical components such as lenses and prisms, in an assembly. The cured resin is therefore characterized as not being reworkable, i.e., polymer network cannot readily be removed from the substrate by thermal or other treatment.
It is an object of the invention herein to provide a curable monomer which provides advantages of conventional acyclic monomers of rapid curing and cured compositions which have high strength and very good moisture resistance, and which, in addition, when cured, are reworkable through thermal decomposition.
The term xe2x80x9creworkable through thermal decompositionxe2x80x9d is used herein to mean thermally degradable at a temperature of not more than 250xc2x0 C. to provide decomposed product that is completely soluble in aqueous NaOH or aqueous NH4OH, thereby to allow repair, replacement, recovery or recycling of components affixed or coated using the cured resin.
To this end, the invention in a first embodiment herein is directed at compounds containing unsaturated aliphatic hydrocarbon moieties which are linked to each other by a tertiary oxycarbonyl containing acyclic moiety, which when cured provide cross-linked networks which are reworkable through thermal decomposition enabling controlled and selective decomposition of the networks. These compounds may be referred to as the monomers herein.
The term xe2x80x9caliphatic hydrocarbonxe2x80x9d is used herein to mean an open chain of carbon atoms which may be straight chain or branched. The term xe2x80x9cacyclicxe2x80x9d is used herein to define the linking moiety as not being and not containing any alicyclic, aromatic or heterocyclic group; this limitation is important since the presence of such group would increase the difficulty of decomposition. The term xe2x80x9ctertiary oxycarbonylxe2x80x9d is used to mean the tertiary ester group 
where Rxe2x80x2 is alkyl. Polymers formed from monomers with tertiary ester group are more readily decomposed than polymers from monomers with primary or secondary ester groups. The tertiary ester group is subject to breakdown into carboxylic acid and alkene at the temperatures contemplated for use for thermal degradation described hereinafter.
In a second embodiment of the invention herein there is provided a photopolymerizable composition comprising compound of the first embodiment herein and a photoinitiation effective amount of a photoinitiator.
The unsaturated aliphatic hydrocarbon moieties of the compounds of the first embodiment herein can be alkenyl, dienyl or alkynyl and are preferably C2-10 alkenyl and very preferably are 
where R is hydrogen or methyl (so the compound is a diacrylate or dimethacrylate).
Preferably the tertiary oxycarbonyl moiety is 
where each Rxe2x80x2 is the same or different and is C1-4 alkyl and where each Rxe2x80x2 very preferably is methyl.
Preferably the linking moiety, i.e., the group linking the two unsaturated aliphatic hydrocarbon moieties, is 
where n ranges from 1 to 30, more preferably from 1 to 6, and very preferably is 3, 4, 5 or 6. The group 
acts as a spacer between two tertiary oxycarbonyl moieties and n may be referred to as the spacer length.
Preferably the compounds of the first embodiment have the structure 
where R is hydrogen or methyl and n ranges from 1 to 30, very preferably from 1 to 6, and most preferably is 3, 4, 5 or 6. Thus, important compounds have the above structural formula where R is hydrogen and n is 4, where R is hydrogen and n is 6, where R is methyl and n is 4 and where R is methyl and n is 6. The monomers of the formula (I) are normally liquids.
The monomers of the formula (I) can be prepared according to the following reaction scheme 
where n and R are as defined above, Et3N is triethylamine, 4-DMAP is 4-(dimethylamino)pyridine, THF is tetrahydrofuran and R.T. is room temperature. The Et3N functions as an acid acceptor. The 4-DMAP functions as a catalyst. The THF is the reaction solvent. The temperatures are used according to the following sequence: The diol, Et3N and 4-DMAP are dissolved in the THF under a nitrogen atmosphere and the solution is cooled to 0xc2x0 C. Then the acryloyl chloride or methacryloyl chloride in THF is added dropwise and after a period of stirring is allowed to warm to room temperature whereupon stirring is continued. We turn now to the diol starting material. 2,5-Dimethyl-2,5-hexanediol (to provide monomer where n=2) is commercially available. The other diols can be prepared by conversion of ester groups of alkanedioic acid esters to tertiary alcohols by a Grignard reaction, e.g., by reaction of xcex1, xcfx89-dimethyl carboxylates, e.g., dimethyl adipate, dimethyl suberate, dimethyl sebacate, dimethyl pimelate, dimethyl azelate, or dimethyl glutarate, with methylmagnesium bromide; this reaction can be carried out by starting with a solution of carboxylate in tetrahydrofuran at 0xc2x0 C., adding the methylmagnesium bromide dropwise, then allowing the reaction to warm to room temperature, stirring and recovering the diol.
We turn now to the embodiment of the invention where a curable composition is provided comprising monomer herein and a photoinitiation effective amount of a photoinitiator. These compositions are prepared by adding photoinitiator to monomer liquid, e.g., in an amount of 0.5 to 10% by weight of the monomer. A preferred photoinitiator is 2-methyl-4xe2x80x2-(methylthio)-2-morpholinopropiophenone which has the formula 
Other photoinitiators include, for example, rose bengal peroxy benzoate, substituted benzophenones and substituted hydroxy benzoins.
We turn now to the curing of the curable compositions of the second embodiment of the invention. Curing is readily carried out by exposing a film (e.g., from 50 nm to 5 mm thick, preferably from 50 nm to 0.1 mm thick) to UV light. The source of the UV light can be, e.g., a UVEXS Model 15609 (123 mW/cm2) at a distance of from 0.1 to 10 cm. Other sources of UV light include, for example, mercury lamps and lasers and other conventional sources of UV light. Significantly higher rates of polymerization were observed for diacrylates than for dimethacrylates, e.g., the polymerization rate for a diacrylate was observed to be up to 20 times the polymerization rate of the corresponding dimethacrylate. The curing time is related to thickness of body of composition being cured. For a film of 0.05 mm, diacrylates cured in 30 seconds and dimethacrylates cured in 240 seconds. A correlation between spacer length and double bond conversion was observed in that the higher double bond conversions were observed for diacrylates where n is 6 or 8 compared to n being 2 or 4 and that higher double bond conversions were observed for dimethacrylates where n is 4, 6 or 8 compared to where n is 2. Where n=2, the Tg (glass transition temperature, i.e., softening point) of the polymer network was about the same for diacrylate and dimethacrylate but in the other cases of n that were observed, the Tg for polymer network diacrylates was less than that for polymer network dimethacrylates. In the case of both diacrylates and dimethacrylates, the Tg of the polymer network decreased as n increased.
Compositions to be cured in a configuration different from a film are cured by initiation of free radicals using a radical initiator. The curable compositions do not contain photoinitiator but rather a radical initiation effective amount of a radical initiator that will initiate free radicals at a temperature less than 150xc2x0 C., such as benzoyl peroxide, lauryl peroxide or azobisisobutyronitrile, and curing is carried out by heating to the temperature where the initiator is functional.
We turn now to the thermal degradation of the cured compositions to residues and the dissolving of the residues. The thermal degradation is carried out by maintaining the cured composition at a temperature ranging from 155xc2x0 C. to 275xc2x0 C., preferably from 180xc2x0 C. to 200xc2x0 C., for 0.1 to 10 minutes, e.g., in a furnace or by using a directed heat probe apparatus, to decompose cured compositions to product which is entirely dissolved in aqueous ammonium hydroxide or aqueous sodium hydroxide. The decomposition provides an anhydride group which is not dissolved in water, methanol or dimethylformamide but which is subjected to ammonolysis in aqueous ammonium hydroxide or hydrolysis in aqueous sodium hydroxide. The dissolving is readily carried out by immersing the assembly where cured composition has been thermally degraded, in a body of aqueous ammonium hydroxide (e.g., 28% aqueous ammonium hydroxide solution) or aqueous sodium hydroxide (e.g., 1N sodium hydroxide solution), for example, for 0.1 to 5 minutes. A lower limit of 155xc2x0 C. is selected because the cured compositions have been found to be chemically stable when maintained at temperatures up to 150xc2x0 C. An upper limit of 275xc2x0 C. is selected because at temperatures exceeding this, the disadvantages of degradation, crosslinking and oxidation of other parts of the assembly may occur. Decomposition rates for diacrylate networks were observed to be higher than decomposition rates for dimethacrylate networks at both 180xc2x0 C. and 200xc2x0 C. When the monomers had the formula (I), maximum decomposition rates (i.e., shortest time for decomposition) was observed for networks formed from monomers where n in formula (I) was 4 or 6. Polymer networks from HDODA are chemically stable at temperatures up to 370xc2x0 C.
The monomers herein are uniquely useful for formulation into curable compositions to function as adhesives or coatings to temporarily hold electronic or optical components on substrates, e.g., to temporarily mount electronic components in the assembly of printed circuit boards or to temporarily fix optical components such as lenses or prisms, whereby the term of affixing may be ended by exposing the assembly to decomposing temperature in an oven and dissolving the residue. The monomers herein are also useful for formulation into curable compositions for more permanent mounting of electronic and other components, and allow repair of inoperative assemblies by replacement of inoperative components or recovering or recycling the operative components of inoperative assemblies.
The synthesis and characterization of monomers of the first embodiment herein and the synthesis and characterization of thermally degradable polymer networks are also described in Ogino, K, et al., Chem. Mater. 10, 3833-3838 (1998) which is incorporated herein by reference and in six pages where the first page is headed xe2x80x9cSynthesis and Characterization of Reworkable Polymer Networksxe2x80x9d which are attached as Appendix B to Provisional Application No. 60/123,263.
As indicated above, this application claims the benefit of U.S. Provisional Application No. 60/123,263; the entire disclosure of U.S. Provisional Application No. 60/123,263 is incorporated herein by reference.